1. Field of Invention
This invention pertains to a molding system. More particularly, this invention pertains to a molding system in which the die faces are formed of thin sheets that are heated and cooled on demand by a fluid outside the mold cavity. Further, the dimensional stability of the thin sheets is maintained by equalizing the pressure between the mold cavity and the fluid cavities on the opposite side of each thin sheet.
2. Description of the Related Art
A common manufacturing method is molding in which a material is placed in a cavity and the material is set or cured to conform to the shape of the cavity. For example, one type of molding process is injection molding, which involves injecting a molten material into a cavity under high pressure. The pressure forces the molten material to flow throughout the cavity. As the molten material cools, it hardens. Temperature control of the injection mold allows the molten material to flow throughout the cavity and to also aid in the setting or curing of the material. The molten material must be maintained above its melting temperature while being injected and filling the cavity. Then the heat from the molten material must be removed to set or cure the material into a molded product.
Typically, a mold is at a lower temperature than the melting temperature of the molten material. As the molten material is injected it cools as heat is transferred to the mold. The injection pressure is maintained at a level sufficient to force the molten material into the cavity. As the molten material begins to cool as it flows, a high injection pressure is required to maintain the flow of the material into the cavity. The high injection pressure requires that the mold be massive and have sufficient structural strength to prevent the mold cavity from deforming when subjected to high injection pressures.
Because molds are massive to withstand the high injection pressures, the molds have high thermal mass. Generally, it is desirable to maintain the temperature of the mold within certain limits to minimize shrinkage and distortion of the material as it sets or cures. Temperature control also ensures uniformity among replicated molded objects. As the mold is used repeatedly, excess heat must be removed quickly to minimize downtime between injecting the molten material for another object. But, the high thermal mass results in difficulties in controlling the mold temperature.
Many ingenious attempts have been made to control the temperature of conventional, massive molds. For example, U.S. Pat. No. 5,762,972 discloses an injection molding system that uses induction heating to quickly heat the mold. U.S. Pat. No. 6,846,445 discloses a mold that uses surface heating generated by surface current due to proximity effect and/or skin effect on the mold insert powered by radio-frequency or high-frequency electrical current. U.S. Pat. No. 6,312,628 discloses a mold that uses low pressure water to cool the mold. The patented apparatus uses water phase change, from liquid to steam, to increase the heat transfer from the mold.
Temperature control is desirable for other molding processes, not just injection molding. For example, resin transfer molding uses a molding material that is exothermic, that is, it gives off heat as the material cures. This excess heat must be removed to avoid undesirable artifacts in the molded object. It is also desirable for the mold to be at an elevated temperature to aid the filling of the mold. For example, U.S. Pat. No. 5,653,907 discloses a shell mold with a hollow support structure with a plurality of intersecting upstanding support ribs supporting a substrate layer that supports a thin, hard, thermally responsive mold shell. The substrate acts as a heat sink to dissipate heat from the shell.